The present invention generally relates to fabrication of semiconductor devices and more particularly to a process for writing a semiconductor pattern on an object such as a semiconductor substrate with a compensation for the proximity effect.
The electron beam lithography is a key process for fabricating advanced semiconductor integrated circuits having a very large integration density. With the use of the electron beam lithography, a device pattern having a line width of less than 0.05 .mu.m can be fabricated easily with an alignment error of less than 0.02 .mu.m. Thus, the electron beam lithography is expected to play a major role in the fabrication of future integrated circuits such as DRAMs having a storage capacity of 256 Mbits or more.
In the fabrication of memory devices, the throughput of production is an essential factor, in addition to the resolution of the device patterning. In this respect, the electron beam lithography that uses a single, focused electron beam for the exposure, is disadvantageous as compared with the conventional optical exposure process that exposes the entire device pattern in one single shot. On the other hand, such a conventional optical exposure process is reaching its limit of resolution, and there is a situation that one has to rely upon the electron beam exposure process for the fabrication of the large capacity memory devices of the future.
Under such circumstances, various efforts have been made for improving the throughput of the electron beam exposure process For example, the inventor of the present invention has previously proposed a so-called block exposure process wherein the device pattern is decomposed into a number of fundamental patterns and the electron beam is shaped in accordance with one of these fundamental patterns. With the use of the block exposure process, one can now achieve a throughput of about 1 cm.sup.2 /sec.
FIG. 1 shows the construction of a conventional electron beam exposure system that uses the technique of block exposure. Referring to the drawing, the electron beam exposure system is generally formed from an electron optical system 100 for producing and focusing an electron beam and a control system 200 for controlling the optical system 100.
The electron optical system 100 includes an electron gun 104 as a source of the electron beam. The electron gun 104 includes a cathode electrode 101, a grid electrode 102 and an anode electrode 103, and produces the electron beam generally in the direction of a predetermined optical axis O in the form of spreading beam.
The electron beam thus produced by the electron gun 104 is passed through a shaping aperture 105a formed in an aperture plate 105. The aperture plate 105 is provided such that the aperture 105a is in alignment with the optical axis O and shapes the incident electron beam to have a rectangular cross section.
The electron beam thus shaped is received by an electron lens 107a that has a focal point coincident to the aperture 105a. Thereby, the incident electron beam is converted to a parallel beam and enters into an electron lens 107b that focuses the electron beam on a block mask 110. It should be noted that the lens 107b projects the image of the rectangular aperture 105a on the block mask 110. As shown in FIG. 2, the block mask 110 carries a number of fundamental patterns 1a, 1b, 1c, . . . of the semiconductor device pattern to be written on the substrate in the form of apertures, and shapes the electron beam according to the shape of the aperture through which the electron beam has passed.
In order to deflect the electron beam passed through the electron lens 107b and address the desired aperture, deflectors 111, 112, 113 and 114 are provided, wherein the deflector 111 deflects the electron beam away from the optical axis O in response to a control signal SM1. The deflector 112 in turn deflects back the electron beam generally in parallel to the optical axis O in response to a control signal SM2. After passing through the block mask 110, the deflector 113 deflects the electron beam toward the optical axis O in response to a control signal SM3, and the deflector 114 deflects the electron beam such that the electron beam travels coincident to the optical axis O in response to a control signal SM4. Further, the block mask 110 itself is provided movable in the direction perpendicular to the optical axis O for enabling the addressing of the apertures on the entire surface of the block mask 110 by the electron beam.
The electron beam thus passed through the block mask 110 is then focused at a point f1 that is located on the optical axis O after passing through electron lenses 108 and 116. There, the image of the addressed aperture on the block mask 110 is demagnified at the point f1. The electron beam thus focused is then passed through a blanking aperture 117a formed in a blanking plate 117 and further focused on the surface of a substrate 123 that is held on a movable stage 126, after passing through electron lenses 119 and 120 that form another demagnifying optical system. There, the electron lens 120 serves for an objective lens and includes various coils such as correction coils 120 and 121 for focusing compensation and astigmatic compensation as well as deflection coils 124 and 125 for moving the focused electron beam over the surface of the substrate 123.
In order to control the exposure operation, the electron beam exposure system of FIG. 1 includes the control system 200, wherein the control system 200 includes memory devices such as a magnetic tape device 201 and magnetic disk devices 202, 203 that are provided to store various data of the device pattern of the semiconductor device to be written. In the illustrated example, the magnetic tape device 201 is used for storing various design parameters, the magnetic disk device 202 is used for storing the exposure pattern data, and the magnetic disk device 203 is used for storing the pattern of the apertures on the block mask 110.
The data stored in the memory devices is read out by a CPU 204 and transferred to an interface device 205 after data decompression. There, the data for specifying the pattern on the block mask 110 is extracted and stored in a data memory 206. The data stored in the data memory 206 is then transferred to a first control unit 207 that produces the foregoing control signals SM1-SM4 and supplies the same to the deflectors 111-114. Further, the control unit 207 produces and supplies a control signal to a mask moving mechanism 209 that moves the block mask 110 in the direction transverse to the optical path O. In response to the deflection of the optical beam by the deflectors 111-114 and further in response to the lateral movement of the block mask 110, one can address the desired aperture on the mask 110 by the electron beam.
The first control unit 207 further supplies a control signal to a blanking control unit 210 that in turn produces a blanking signal for shutting off the electron beam. This blanking signal is then converted to an analog signal SB in a D/A converter 211 and the analog signal SB is supplied to a deflector 115 that causes a deflection of the electron beam away from the optical axis O. In response to this, the electron beam misses the blanking aperture 117a and disappears from the surface of the substrate 123. Further, the control unit 207 produces a pattern correction data H.sub.ADJ and supplies the same to a D/A converter 208. The D/A converter 208 in turn produces a control signal S.sub.ADJ and supplies the same to a deflector 106 that is provided between the electron lens 107a and the electron lens 107b. Thereby, one can modify the shape of the electron beam that have passed through the addressed aperture in the mask 110. This function is used when the desired shape of the electron beam is different from the shape given by the apertures on the block mask 110.
The interface device 205 further extracts and supplies the data for controlling the movement of the electron beam on the surface of the substrate 123 to a second control unit 212. In response thereto, the control unit 212 produces a control signal for controlling the deflection of the electron beam on the surface of the substrate 123 and supplies the same to a wafer deflection control unit 215 that in turn produces and supplies deflection control signals to D/A converters 216 and 217. The D/A converters 216 and 217 in turn produce drive signals SW1 and SW2 for driving the deflectors respectively and supply the same to the deflectors 124 and 125 for causing the deflection of the electron beam. Thereby, the position of the stage 126 is detected by a laser interferometer 214 and the wafer deflection control unit 215 modifies the output deflection control signals and hence the drive signals SW1 and SW2 according to the result of measurement of the stage position by the laser interferometer. Further, the second control unit 212 produces a control signal that causes a lateral movement of the stage 126.
FIGS. 3(A) and 3(B) show the exposure of various patterns on the substrate 123 achieved by the apparatus of FIG. 1.
Referring to FIG. 3(A), the drawing shows the energy or dose of the electron beam supplied to an electron beam resist that covers the surface of the substrate for a case where the exposed device pattern designated as P.sub.1 has a pattern density .alpha. close to 100%. The pattern density herein means the percentage of the region of the substrate that is exposed by the electron beam. In FIG. 3(A), the threshold of exposure energy is represented as TH. When the dose of the electron beam exceeds the foregoing threshold TH, the exposure of the electron beam resist occurs. On the other hand, when the dose does not reach the threshold TH, no exposure is made.
In the exposure of FIG. 3(A), it should be noted that there is a substantial background exposure as represented by .beta., which is caused by the backscattering of electrons from the substrate. Such a background exposure is known as "proximity effect." When such a background exposure occurs, the dose of the electron beam for the device pattern P.sub.1 increases inevitably as indicated in FIG. 1(A). As a natural consequence, such a background exposure does not occur in small isolated patterns such as a pattern P.sub.2 where the backscattering of the electrons is small. Thereby, there occurs a difference in the dose of exposure between the region where a dense pattern is exposed and the region where a small isolated pattern is formed.
A similar change of dose occurs also between the device patterns having a large pattern density such as the pattern P.sub.1 and the device patterns having less dense pattern density such as a pattern P.sub.3 shown in FIG. 3(B). The device pattern P.sub.3 of FIG. 3(B) has a pattern density of 50%, for example, and there occurs a backscattering of the electrons with a magnitude represented as .alpha..beta..
Under such a situation, it will be understood that the variation of exposure, caused by the backscattering of electrons, has to be compensated for such that there is a uniform background exposure. Otherwise, there occurs a case where small isolated patterns such as the pattern P.sub.2 of FIG. 3(A) are not exposed.
In order to achieve the desired compensation for the background exposure, a so-called ghost exposure process is proposed by Owen et al. (Owen, G., Rissman, P. and Long, M. F., Application of the Ghost proximity effect correction scheme to round beam and shaped beam electron lithography systems, J. Va. Sci. Technol. B3(1), January/February 1985), which is incorporated herein as reference.
FIGS. 4(A)-4(C) show the principle of the foregoing ghost exposure process, wherein FIG. 4(A) shows the exposure corresponding to FIGS. 3(A) and 3(B) where there is no compensation of the proximity effect. There, it will be noted that the dose or level of exposure changes in the patterns P.sub.1, P.sub.2 and P.sub.3 because of the different level of backscattering and hence the background exposure. Further, it should be noted that there occurs a change in the level of dose even in the same device pattern between the region located at the center of the pattern and the region located at the margin of the pattern. In other words, there appears a shoulder at the marginal part of the exposed device pattern.
In order to compensate for the non-uniform background exposure, the ghost exposure process uses a diffused or defocused electron beam that is produced in correspondence to the inversion of the exposed device patterns as shown in FIG. 4(B). By superposing the ghost exposure of FIG. 4(B) on the exposure of the real pattern of FIG. 4(A), one can obtain a uniform background level throughout the patterns P.sub.1, P.sub.2 and P.sub.3 as shown in FIG. 4(C). Thereby, the problem of undesirable variation of the dose is successfully eliminated.
In the exposure process of FIG. 4(C), however, there exists an obvious disadvantage in that the ghost exposure has to be conducted for the entire surface of the substrate, regardless of the fact that there is a device pattern or not. Thereby, the efficiency of exposure is substantially reduced.
In the exposure process of FIG. 4(C), there exists another, more serious problem in that the fine device patterns such as the pattern P.sub.3 tend to be exposed excessively because of the increased background level. More specifically, the ghost exposure of FIG. 4(B) inevitably sets the background level of exposure of the device pattern P.sub.3 substantially identical with the background level of the device pattern P.sub.1 that fills or occupies an area of the substrate continuously with the pattern density .alpha. of about 100%. In FIG. 4, it should be noted that the threshold dose of the electron beam resist is represented by the broken line designated as TH. When the fine device pattern such as the pattern P.sub.3 is subjected to the excessive exposure, there arises a problem in that the size of the individual spot or line that forms the pattern P.sub.3 tends to be enlarged. Thereby, the proper exposure of fine or minute device patterns, which are essential for the fabrication of large storage memory devices, is no longer possible.
FIG. 5 shows the foregoing problem of increase of size of the exposed device pattern caused by the excessive dose of the exposure. For the sake of illustration, FIG. 5 is exaggerated significantly.
Referring to FIG. 5, the size of the exposed pattern such as a dot or line changes in relation to the dose of exposure and the threshold level of the electron beam resist. In the case where the level of dose is close to the threshold of exposure as represented by TH.sub.1, one obtains a pattern size D.sub.2 that is close to the desired device pattern D.sub.1. On the other hand, when the level of dose exceeds the threshold of exposure substantially as represented by TH.sub.2, one obtains a much larger pattern size D.sub.3. Thus, the exposure of the device patterns such as the pattern P.sub.3 by the ghost exposure inevitably decreases the resolution of the exposed pattern. In the worst case, the desired device pattern cannot be exposed.
One may think that such a problem of ghost exposure would be avoided by simply increasing the dose for those parts of the device pattern wherein the dose is insufficient such as the marginal part of the device. FIGS. 6(A)-6(D) show an example of the compensation for the insufficient dose according to the above mentioned principle, wherein FIG. 6(A) shows the device pattern to be exposed and FIG. 6(B) shows the corresponding background exposure level caused by the proximity effect. As a result of the reduced backscattering of electrons at the marginal part, the device pattern of FIG. 6(A) takes a form shown in FIG. 6(C) wherein the size of the individual pattern elements forming the device pattern of FIG. 6(A) is reduced at the marginal part. More specifically, it will be noted that the elongated rectangular pattern element forming the device pattern of FIG. 6(A) is deformed into a trapezoidal shape because of the reduced dose at the outer edge part of the device pattern. Thus, even when the dose of the exposure is increased in correspondence to the marginal part for compensating for the reduced dose in these parts, the trapezoidal shape of the Pattern element is not rectified. See FIG. 6(D) that shows the device pattern exposed on the substrate with such a compensation process. In order to obtain a correct rectangular shape of the exposed pattern element, one has to divide the pattern element into a plurality of subelements and change the dose in correspondence to each subelement as shown in FIG. 6(D) by the asterisk, solid circle and the solid square However, such a subdivision of the pattern element is contradictory to the improvement of throughput of exposure achieved by the block exposure process.